If you want to be a loser, be fat. And please — take the statement less as an admonishment than a simple fact of life in America. As a people, we Americans dislike fat, and since fat collects on people, we dislike fat people.

We can't escape the fact that we discriminate against overweight people. Try getting the choice job if you have trouble fitting into your interview chair. You're seeing fewer fat executives these days. Gluttony is out, slim is in. Countless executive fitness journals lay down guidelines for the welfare of our business and civic leaders. President Reagan regularly trains with weights. So does Gary Hart. Former President Carter jogs every day.

It isn't that we don't like fat people, it's simply that a body loaded down with fat runs counter to the national passion for slimness. Fat people, we believe, don't fit in. Their gears don't mesh with ours. They are flawed, we think. They are self-indulgent, have no control. We hate that. Don't they know what they're doing to themselves?

And fat is the perfect target for prejudice. With almost 100 million Americans overweight, we've got a lot of people to condemn.

Much recent research, however, supports the belief that fat people, no matter how determined they are to get thin, are waging a battle they are not likely to win. Whether their problem is physiological or psychological, they are handicapped, and even their effort to overcome it is akin to pulling themselves up by their bootstraps. They know it's better to be thin because the more biologically lucky ones get all the breaks.

But obesity doesn't have to be permanent, although the track record for diets would make you think so. People are beginning to lick the fat problem. Here are some myths and scientific facts that can help support the effort:

MYTH: Fat is not a metabolic problem.

FACT: Fat people have a different metabolism. Experiments have shown that some people can eat excessively and not gain weight while others will make huge weight gains. Researchers believe this may be clue to a greater amount of active brown fat in thin people than in fat people. Only about 1% of the body's fat consists of brown fat. The rest is yellow fat that is relatively inert, metabolically speaking.

Hormones that direct energy consumption are more active in thin people. Fat people store calories easily and burn them sparingly. Fat people may overeat because certain hormones carry faulty appetite messages to the brain.

The sodium-potassium pump works more efficiently in thin people. Each body cell has its own pump and works continually to maintain a sodium-potassium balance. The process consumes many calories. The pump works less efficiently in fat people and results in more stored calories.

Being fat establishes metabolic functions that keep you fat. On the upbeat side, there is undeniable evidence now that exercise and proper nutrition reverses and normalizes these functions.

MYTH: Blaming your obesity on family tendencies is a lame excuse.

FACT: If your parents are fat, you have up to an 80% chance of becoming a fat adult. There is scientific disagreement on such inherited tendencies. Environment may be to blame because of the eating and activity patterns laid down by your parents.

Any rapid weight gain causes an increase in the number of fat cells, which, once termed, stay with you for life. A weight-loss regimen may reduce them, but they secrete an enzyme called lipoprotein lipase that tells your body to restore them to full size. Staying thin after reducing is not easy with millions of extra fat cells demanding former status.

MYTH: The emotional and psychological makeup of fat people causes them to eat more.

FACT: Researchers have found no difference between thin and fat people psychologically and emotionally. What is clear, however, is that overeating and inactivity keep you fat and getting fatter.

MYTH: Fat people have no willpower.

FACT: Many fat people diet religiously over long periods. Some lose weight and other's don't, even at near-starvation levels.

Exercise remains the single most effective stabilizing factor in losing and maintaining bodyweight. If you increase your exercise time and reduce your calorie intake comfortably, weight can be lost slowly but permanently. Never, never depend on diet alone. Get your sodium-potassium pump in good working order and establish the self-image that is essential to the success of your fitness lifestyle.